Centrosomes are composed of two centrioles and pericentriolar material, including centrosomal proteins. Centrosomes are known to play a role in cell division and microtubule nucleation, and may also be involved in cytokinesis completion (Gromley et al, J Cell Biol. 2003 May 12; 161(3):535-45. Epub 2003 May 5; US PGPub No. 2005/0208058) and membrane trafficking; centrosomal proteins are required for secretory vesicle mediated cytokinesis (Gromley et al., Cell. 2005 Oct. 7; 123(1):75-87).
The centrosome of vertebrate cells and its yeast equivalent, the spindle pole body, have been well recognized as microtubule organizing centers (MTOC) in eukaryotic cells. Centrosomes play an important role in nucleating and organizing of microtubules in interphase and forming the mitotic spindle necessary for proper segregation of DNA during mitosis. More recently, centrosome proteins have been shown to provide a molecular ‘scaffold’ to enable cell polarization and to localize proteins involved in certain signaling pathways. In addition, centrosome proteins comprise the structural underpinning for formation of the primary cilium, an organelle that ‘senses’ the cellular milieu (Jurczyk et al., J. Cell. Biol. 166(5):637-43 (2004). In each case, these diverse functions derive from a concentrated confluence of centrosome proteins at a single cytosolic location. However, it is little appreciated that fully half of the total centrosome protein is more diffusely distributed within the cytosol, yet its exact localization and functional importance there is not understood.
The centrosome is a structurally complex organelle consisting of two centrioles at right angles to each other, surrounded by the pericentriolar material (PCM). Numerous proteins have been identified in the centrosome, each with diverse functions implicating potential involvement in a multitude of cellular processes. Pericentrin and γ tubulin are integral centrosome proteins that play an important role in microtubule nucleation and organization, cell cycle progression and ciliogenesis (Doxsey et al., Cell. 76:639-650 (1994); Oakley and Oakley, Nature. 338(6217):662-4 (1989); Jurczyk et al., (2004) supra; Mikule et al., Nature Cell Biology. 9(2):160-170 (2007)). Pericentrin consists of multiple isoforms, three of which have been characterized: pericentrin B (360 kDa), pericentrin A (255 kDa), and pericentrin S (250 kDa) (Flory and Davis, Genomics. 82:401-5 (2003); Doxsey et al., (1994), supra; Miyoshi et al., Biochem. Biophys. Res. Comm. 351:745-749 (2006)). This large molecule interacts with numerous proteins and protein complexes including γ tubulin ring complex (Zimmerman et al., Mol. Biol. Cell. 15:3642-57 (2004)) and cytoplasmic dynein (Purohit et al., J. Cell. Biol. 147:481-491 (1999)). Gamma tubulin is indispensable for microtubule nucleation, and it is recruited to microtubule nucleating sites from the cytosol. Indeed, the centrosome pool of γ tubulin is freely exchangeable with the cytosolic pool, except for the fraction that associates with the centrioles (Khodiakov and Rieder, J. Cell. Biol. 146(3):585-96 (1999)).